Alumina insulation for coating implantable components and other microminiature devices

ABSTRACT

A protective, biocompatible coating or encapsulation material protects and insulates a component or device intended to be implanted in living tissue. The coating or encapsulation material comprises a thin layer or layers of alumina, zerconia, or other ceramic, less than 25 microns thick, e.g., 5-10 microns thick. The alumina layer(s) may be applied at relatively low temperature. Once applied, the layer provides excellent hermeticity, and prevents electrical leakage. Even though very thin, the alumina layer retains excellent insulating characteristics. In one embodiment, an alumina layer less than about 6 microns thick provides an insulative coating that exhibits less than 10 pA of leakage current over an area 75 mils by 25 mils area while soaking in a saline solution at temperatures up to 80° C. over a three month period.

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/033,637, filed Dec. 20, 1996.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to very thin layers of electricalinsulation that may be used to coat and protect microminiaturecomponents and devices that are intended to be implanted in livingtissue and/or to maintain electrical leakage of such components/deviceswithin acceptable limits, e.g., less than 1 μA/cm² when the componentsand/or devices are submerged in water or salt water. More particularly,the invention relates to the use of alumina or aluminum oxide as a safe,biocompatible, coating material that provides a reliable, protective andinsulative layer or coating for components, or devices comprised ofcomponents, wherein the insulating layers can be made extremely thin, onthe order of microns, yet wherein the electrical leakage through thethin insulative layer (when the coated component or device is implantedor otherwise immersed in a saline solution or in distilled water) isless than about 1 μA/cm² (or less than about 12.1 nA for an area of0.075 inches×0.025 inches, corresponding to an area of 0.1905 cm by0.0635 cm).

[0003] The use of alumina as a thick insulator for use with implantabledevices has previously been disclosed, for example, in U.S. Pat. Nos.4,940,858 and 4,678,868 assigned to Medtronic, Inc. In theseapplications, however, the alumina insulator is very thick and is usedonly as part of the feedthrough for the implantable device and is oftencarried by a metal ferrule. Such use of alumina (or other ceramic) as aninsulator requires a relatively thick layer. Many materials work well asan insulator when put down in a thick layer, e.g., in a layer thickerthan 25 microns (where 1 micron=1×10⁻⁶ meter). But all such materials,except as discussed herein, typically leak at a rate greater than about1 μA/cm². Applicants invention, as set forth below, uses a nonconductiveceramic, such as alumina, in very thin layers, e.g., less than about 25microns.

[0004] It is also known to use the ceramic alumina as a case materialfor an implanted device as disclosed in U.S. Pat. No. 4,991,582,incorporated herein by reference. Again, however, the alumina, whilecomprising a material that is biocompatible (and is thus not harmful to,and is not harmed by, living tissue and fluids wherein it is implanted),is relatively thick, e.g., greater than 25 microns.

[0005] A problem with the related art is that the thickness of theinsulation needed for implantable devices is typically on the order ofabout several millimeters thick. None of the related art, to applicant'sknowledge, has heretofore achieved an insulating layer with very smalldimensions and free of micro-holes. The presence of a micro-hole, or“pin-hole”, destroys the insulating properties which may lead toeventual failure of the implantable device.

[0006] Further, some components or devices which need to be implanted inliving tissue, such as magnets, are susceptible to extremely hightemperatures, i.e., extremely high temperatures may damage or destroysuch components. When such components or devices must be implanted, itis important therefore that whatever coating or encapsulating materialis used to coat them be one that can be applied without subjecting thecomponent or device to extremely high temperatures. That is, the coatingor application process must not subject such components to extremelyhigh temperatures.

[0007] It is seen, therefore, that what is needed is a way to utilize avery thin layer of a suitable insulating material, such as alumina(aluminum oxide), zirconia (zirconium oxide), or alloys of aluminaand/or zirconia, at relatively low temperatures, as a coating to cover,insulate and/or encapsulate any type of component or device that must beimplanted, thereby effectively rendering such coated component or devicebiocompatible and safe for implantation. In particular, it is seen thatwhat is needed is a very thin insulative coating that can be applied atrelatively low temperatures for the purpose of insulating electricalconnections on implantable devices and other microminiature devices, orfor coating non-biocompatible components (thereby making the coatedcomponent biocompatible) wherein the coating can be as thin as about{fraction (1/1000)} of an inch or less yet still maintain the electricalleakage through the insulator at or below acceptable levels.

[0008] The present invention addresses the above and other needs.

SUMMARY OF THE INVENTION

[0009] The present invention provides a protective, biocompatiblecoating or encapsulation material that may be applied to a component ordevice intended to be implanted in living tissue. The coating orencapsulation material comprises a thin layer or layers of alumina,zirconia, and/or alloys of alumina and/or zirconia. Advantageously, athin alumina or zirconia layer applied in accordance with the presentinvention may be applied at relatively low temperature. Once applied,the coating provides excellent hermeticity, and prevents electricalleakage, while retaining a microminiature size. The layer of alumina orzirconia insulation can be made as thin as about {fraction (1/1000)} ofan inch (Note: {fraction (1/1000)} inch=0.001 inch=1 mil=25.4 microns)or less while still retaining excellent insulating characteristics. Forexample, in accordance with one aspect of the present invention, analumina coating having a thickness that is less than about 5-10 micronsprovides an insulative coating that exhibits less than about 12 nA ofleakage current over an area 75 mils by 25 mils while soaking in asaline solution at temperatures up to 80° C. over a three month period.

[0010] Advantageously, the invention may be used to encapsulate or coat(and thereby insulate) passive electrical and/or magnetic components,such as resistors, capacitors, inductors, wire, conductive strips,magnets, diodes, etc., and/or active electrical components, such astransistors, integrated circuits, etc., and/or assemblies orcombinations of such passive and/or active components. Because thecoating layer can be made extremely thin, yet still provide the neededinsulative properties required for an implanted component or device, theoverall size of such components or devices does not increasesignificantly from the normal size (non-implanted size) of suchcomponents or devices. For many applications, e.g., as taught in U.S.Pat. No. 5,193,539, incorporated herein by reference, a completeimplanted device, comprised of many different components, may be coatedand maintained at a microminiature size. For other applications, e.g.,the implantation of one or more permanent magnets, such magnets may becoated with the alumina or zirconia coating, thereby effectivelyhermetically sealing the magnets in an alumina or zirconiumencapsulation that renders the magnets suitable for direct implantationin living body tissue at a desired location.

[0011] It is an object of the invention to provide a biocompatible,thin, insulative coating that is easy to apply to a wide variety ofdifferent shapes and sizes of components and devices, and that onceapplied provides excellent insulative properties for the coveredcomponent or device over a long period of time, thereby allowing thecovered component or device to be safely implanted in living tissue forlong periods of time.

[0012] It is a further object of the invention, in accordance with oneaspect thereof, to provide a biocompatible, insulative coating that maybe applied to implantable components or devices of various shapes andsizes, and wherein the coating is: (1) less than about 10 microns thick;(2) submersible for long periods of time in water or saline solution orany other conductive fluids; (3) made from alumina, zirconia or alloysof alumina and/or zirconia, or other substances having properties thesame as or similar to alumina, zirconia and/or alloys of alumina and/orzirconia; (4) amenable to being applied using a batch process, e.g., aprocess wherein 1000 or more devices or components may be coated at thesame time using the same process, such as an evaporative coating, vapordeposition, or ion-beam deposition (IBD) process; and/or (5) extremelystrong in the lateral direction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other aspects, features and advantages of thepresent invention will be more apparent from the following moreparticular description thereof, presented in conjunction with thefollowing drawings, wherein:

[0014]FIG. 1A illustrates an electronic device that has been coated witha thin insulative layer in accordance with the present invention;

[0015]FIG. 1B shows an enlarged side view of a portion of the substrateof FIG. 1A so as to depict the alumina layer thereon;

[0016]FIG. 2 shows a component coated with a thin insulative layer inaccordance with the invention, thereby rendering such component (whichwithout the coating may be non-bio-compatible and non-implantable) bothbiocompatible and implantable; and

[0017]FIG. 3 is a flow chart that depicts, in general steps, the methodof applying a coating in accordance with the invention.

[0018]FIG. 4 schematically depicts one method that may be used to coatfive of six sides of an object with an insulative coating in accordancewith the present invention;

[0019]FIG. 5 is a more detailed flow chart of a preferred method ofapplying a coating to an object in accordance with the invention; and

[0020]FIG. 6 is a flow chart that illustrates a preferred test used todetermine how many layers of a coating need to be applied, i.e., howthick of a coating is needed, in order to provide a coating free ofmicro-holes (also called “pinholes”).

DETAILED DESCRIPTION OF THE INVENTION

[0021] The following description is of the best mode presentlycontemplated for carrying out the invention. This description is not tobe taken in a limiting sense, but is made merely for the purpose ofdescribing the general principles of the invention.

[0022] In the description of the invention herein, reference isfrequently made to a “layer of alumina” or to an “alumina insulationlayer” as the preferred material for the coating or layer that comprisesthe invention. Alumina, as is known in the art, comprises a shorthandnotation for aluminum oxide, Al₂O₃. It is to be understood that all suchreferences to an insulating layer or coating made from “alumina” alsoapply to an insulating layer made from other suitable substances, suchas magnesium oxide, zirconium oxide (zirconia), alloys of alumina and/orzirconia, and the like. In general, such oxides may be referred to asceramics.

[0023] An alumina insulation layer or coating for microminiature orother devices is applied by depositing one or successive layers ofalumina to electrical connections and/or other electronic circuitry orcomponents. In some cases, the component or object to be coated maycomprise an IC chip by itself.

[0024] Each insulating layer applied is preferably made by depositingaluminum oxide (“alumina”), or other suitable insulating material, so asto coat the desired surface of the component or device.

[0025] A common application for the alumina insulating coating of thepresent invention is to insulate or encapsulate the entire surface of ahybrid integrated circuit 12 formed on a ceramic substrate 14, once thehybrid integrated circuit 12 has been formed, with an insulative layer16, as illustrated in FIG. 1A. In FIG. 1A, by way of example, thesubstrate 14 may have a capacitor 18 and an integrated circuit chip 20mounted thereon, both of which are also coated with the insulative layer16. Depending upon the function of the hybrid circuit 12, an electrode22 may also be connected thereto via a coated wire 24. Also, to providea return path from the electrode 22, a portion of the layer 16 thatcovers on end of the substrate 14 may be removed, thereby exposing areturn electrode 26.

[0026] For other applications, the alumina insulating coating is appliedto insulate or encapsulate just the integrated circuit (IC) chip 20 byitself. Any electrical connections that may need to be made to the ICchip, e.g., via an insulated wire, may be made prior to application ofthe insulating coating. In such instance, the IC 20 once coated couldthen be implanted directly into living tissue yet still perform itsintended function.

[0027] The insulative layer 16 is very thin, having a thickness “t” onthe order of 5-25 microns. Thus, the layer 16 is not readily visible inFIG. 1A, but is represented in the enlarged and magnified side view ofFIG. 1B.

[0028] Alternatively, an insulating coating 16′ may be used to insulateselected metal traces 28 and 30, or components 32 and 34, mounted on orto a ceramic substrate 14′ of a hybrid integrated circuit 12′, whileother components, such as electrode 36, or some portions of the surfaceof the substrate 14′, are not coated or encapsulated, as illustrated inFIG. 2A. In FIG. 2A, those components or surface areas not to be coatedwith the layer 16′ may be masked using conventional techniques at thetime the coating 16′ is applied.

[0029] In general terms, and for applications where a hybrid circuit, anIC chip, or other device is to be coated with alumina in accordance withthe encapsulation/coating process of the present invention, the stepsfollowed by the invention are illustrated in FIG. 3 and may besummarized as:

[0030] (1) Atomically cleaning an insulating substrate or IC chip (ifnecessary) with a plasma cleaning, or equivalent, process (block 102 ofFIG. 3). Note: if an IC chip is being coated by itself, and if the ICchip has not yet left its clean fabrication environment, this step maynot be needed. The insulating substrate, when used, may be made from, oralready coated with, successive layers of alumina or other suitableinsulating material, such as magnesium oxide or zirconia.

[0031] (2) Depositing metallized patterns of a suitable conductivematerial on one or more of the exposed surfaces of the substrate (block104). The metallized patterns are preferably deposited or etched on thesubstrate using conventional thin film deposition, painting ormetallized etching techniques, as are common in the printed circuitboard and integrated circuit fabrication arts. These patterns are usedto make desired electrical connections between components of thecircuit.

[0032] (3) Depositing a layer of titanium on the metallized portions ofalumina substrate (block 106). Typically, such layer of titantium willbe about 300 Å thick.

[0033] (4) Depositing additional layers of alumina, using anion-enhanced evaporative sputtering technique, or ion beam deposition(IBD) technique, over the entire surface of the substrate including themetallized traces. Using an IBD technique, for example, one applicationof alumina may lay down a layer of alumina that is only 1-2 micronsthick. Through application of several such layers, an alumina coatingmay thus be formed of sufficient thickness to provide the desiredinsulative (leakage current) and encapsulation (hermeticity) properties.Advantageously, the deposited alumina coating (comprising a plurality ofdeposited layers) need only be 5-10 microns thick.

[0034] Various techniques may be used to apply the alumina insulationover the device or component that is to be insulated. A preferredtechnique, for example, is to use an ion beam deposition (IBD)technique. IBD techniques are known in the art, as taught, e.g. in U.S.Pat. No. 4,474,827 or 5,508,368, incorporated herein by reference.

[0035] Using such IBD techniques, or similar techniques, the desiredalumina layer may be deposited on all sides of an object 15 asillustrated in FIG. 4. As seen in FIG. 4, the object 15 is placed on asuitable working surface 40 that is rotatable at a controlled speed. Theworking surface 40, with the object 15 thereon, is rotated while a beam42 of ions exposes the rotating surface. Assuming the object 15 has sixsides, five of the six sides are exposed to the beam 42 as it rotates,thereby facilitating application of the desired layer of alumina ontothe five exposed sides of the object. After sufficient exposure, theobject is turned over, thereby exposing the previously unexposed side ofthe object to the beam, and the process is repeated. In this manner,four of the sides of the object 15 may be double exposed, but suchdouble exposure is not harmful. Rather, the double exposure simplyresults in a thicker coating of alumina on the double-exposed sides.

[0036] Other techniques, as are known in the art, may also be used toapply the alumina coating to the object.

[0037] The steps typically followed in applying a coating of alumina toan object are illustrated in the flow chart of FIG. 5. As seen in FIG.5, these steps include:

[0038] (a) Sputtering a layer of titanium of about 300 Å thick over anymetal conductor or other object that is to be coated with the alumina(block 110 of FIG. 4).

[0039] (b) If selective application of the alumina to the object is tobe made (YES branch of block 112), spinning a photosensitive polyamideonto a ceramic hybrid substrate, or other component to be encapsulatedwith the alumina or other substance (block 114).

[0040] (c) Applying a mask that exposes those areas where Alumina is notto be applied (block 116).

[0041] (d) Shining ultra violet (UV) light through the mask topolymerize the polyamide (block 118). Where the UV light illuminates thepolyamide is where aluminum oxide will not be deposited. Thus, thepolymerization of the polyamide is, in effect, a negatively actingresist.

[0042] (e) Developing the photoresist by washing off the unpolymerizedpolyamide with xylene (block 120), or an equivalent substance. Once theunpolymerized polyamide has been washed off, the ceramic (or othercomponent) is ready for aluminum oxide deposition.

[0043] (f) If selective application of the alumina is not to be made (NObranch of block 112), i.e., if alumina is to be applied everywhere, orafter washing off the unpolymerized polyamide (block 120), depositingaluminum oxide to a prescribed thickness, e.g., between 4 and 10microns, e.g., 6 microns, over the object using ion enhanced evaporation(or sputtering), IBD, or other suitable application techniques (block122).

[0044] (g) During application of the coating, rotate and/or repositionthe object as required (block 124) in order to coat all sides of theobject, e.g., as shown in FIG. 4, with a coating of sufficientthickness. This step may require several iterations, e.g., incrementallydepositing a thin layer of alumina (block 126), checking the layer forthe desired thickness or properties (block 127), and repeating therepositioning (block 124), depositing (block 126), and checking (block127) steps as required until a desired thickness is achieved, or untilthe coating exhibits desired insulative and/or hermeticity properties.

[0045] (h) Breaking or scribing the aluminum oxide that resides over thepolyamide, if present, with a diamond scribe, or laser, controlled by acomputerized milling machine (block 128). This permits a pyranasolution, explained below, to set under the oxide for subsequent liftoff of the aluminum oxide.

[0046] (i) Lifting off the polyamide and unwanted aluminum oxide aftersoaking the substrate in pyrana solution (H₂SO₄×4+H₂O₂×2 heated to 60°C.) (block 130). Soaking should occur for 30 to 60 minutes, depending onthe thickness of the polyamide layer.

[0047] For some applications, the device to be coated may comprise anentire IC chip or a permanent magnet, e.g., a small ceramic magnet. Whenan IC chip or a magnet is to be coated with alumina, a similar processto that described above is followed, except that there are no metaltraces or pads that need to be deposited or covered. Rather, the entirechip or magnet is coated with one or more layers of alumina.

[0048] Leakage tests and voltage breakdown tests, when applicable, mayalso be performed in conventional manner in order to determine theinsulative and/or sealing properties of the coating. Typically, thedevice or component is immersed in a saline solution representative ofliving body tissue. Next, a voltage is applied between a metal tracecovered by the alumina and a platinum black electrode, or otherreference electrode, positioned proximate the covered device. Thevoltage is slowly increased while watching/monitoring the current drain.The voltage increase is stopped and measured at the point wherebreakdown occurs. Leakage current is measured by keeping the appliedvoltage at a constant value and monitoring the current drain.

[0049] A useful test for determining how thick the alumina coating mustbe to eliminate micro-holes, or pinholes, is shown in the flow diagramof FIG. 6. As seen in FIG. 6, a first step is to apply a layer of purealuminum to a test object (block 140). This layer of pure aluminumserves as a base layer. Then, n layers of a suitable oxide, such asalumina, are applied over the base layer, where n is an integer of frome.g., 1 to 5. Each of these n oxide layers are applied in a controlledmanner, using, e.g., IBD techniques, so that each deposited layer has athickness that is more or less consistent, e.g., 1-2 microns. Afterapplication of n layers of alumina (or other ceramic), the coated deviceis dipped in an acid (block 143). If any pinholes are present in thecoating, then the acid immediately starts to react with the aluminumbase layer, leaving a very detectable ring. Thus, by performing a simplevisual inspection of the device (block 144), one can easily determinewhether there is any evidence of pinholes (block 146). If evidence ofpinholes is seen (YES branch of block 146), then that is evidence thatthe n layers of alumina that were deposited did not create asufficiently thick coating (block 150). Thus, the value of n isincreased (block 152), and the test is repeated. If no evidence ofpinholes is seen (NO branch of block 146), then that is evidence thatthe alumina coating is sufficiently thick.

[0050] Generally, 4-6 layers of alumina, creating a total coatingthickness of 5-10 microns, is sufficient to reduce leakage current toless than about 6 pa. For desired hermeticity, at least about 6 layersof alumina are typically required.

[0051] It is to be emphasized that while using alumina in an implanteddevice is not new, depositing extremely thin layers of alumina, e.g., 5to 10 microns thick, over components or devices to be implanted, andthen relying on such thin layer of alumina to act as an insulative layeror coating, is new, and has produced surprising and unexpected resultsrelative to its insulative properties.

EXAMPLE

[0052] A test specimen that included a plurality of 75 mil by 25 mil and75 mil by 5 mil metallized pads deposited on an alumina substrate wasconstructed using conventional techniques. The plurality of metallizedpads are separated from one another by a distance of about 2.0-2.5 mils.A layer of alumina insulator approximately 5-6 microns thick wasdeposited on and between the metallized pads using an ion-enhancedevaporative sputtering technique. The ion-enhanced evaporativesputtering was performed in an evacuated chamber at a moderatetemperature of about 60-100° C., and allowed to cure for approximately0.5-4 hours. The test specimen was subsequently submersed in a salinesolution at 87° C. for three months. Leakage current between themetallized pads and the saline solution was measured and did not exceed10 pA across the 6 micron size insulating layer. In addition leakagecurrent between each metallized pads did not exceed 10 pA across the2.0-2.5 mil spacings.

What is claimed is:
 1. An implantable component comprising: a substrate;at least one electrical component mounted on said substrate, and abiocompatible, insulative coating applied to selected portions of thesubstrate and the at least one electrical component, said-insulativecoating comprising: a layer of alumina, zirconia, magnesium oxide, oralloys of alumina, zirconia or magnesium oxide that is less than about25 microns thick, said coating exhibiting insulative properties thatmaintain electrical leakage through said coating to less than about 1μa/cm².
 2. An implantable component comprising: an electrical component;and a biocompatible, insulative coating applied to said electricalcomponent, said insulative coating comprising: a layer of alumina,zirconia, magnesium oxide, or alloys of alumina, zirconia or magnesiumoxide that is less than about 25 microns thick, said coating exhibitinginsulative properties that maintain electrical leakage through saidcoating to less than about 1 μa/cm².
 3. A biocompatible, insulativecoating applied to a component, said insulative coating comprising alayer of alumina, zirconia, magnesium oxide, or alloys of alumina,zirconia or magnesium oxide, deposited on said component so as to forman insulative and protective coating that is less than about 25 micronsthick, said coating exhibiting insulative properties that maintainelectrical leakage through said coating to less than about 1 μa/cm²,whereby said component with said coating applied thereto is implantable.4. The biocompatible, insulative coating of claim 3 wherein said coatingis less than about 10 microns thick.
 5. The biocompatible, insulativecoating of claim 3 wherein said component comprises a hybrid circuithaving a substrate and a least one electrical component mounted on saidsubstrate.
 6. The biocompatible, insulative coating of claim 3 whereinsaid component comprises an integrated circuit chip.
 7. Thebiocompatible, insulative coating of claim 3 wherein said componentcomprises a magnet.
 8. A method of placing a sealed, protective andinsulative coating on an object to be submersed in a conductive medium,such as living tissue, said method comprising: (a) depositing a layer oftitanium on a surface of the object that is about 300 Å thick; and (b)depositing a layer of alumina, zirconia, magnesium oxide, alloys ofalumina, zirconia or magnesium oxide over the surface of the object thatis about 5-10 microns thick.
 9. The method set forth in claim 3 whereinstep (b) comprises depositing, using an ion-enhanced evaporativesputtering technique, successive layers of alumina, zirconia, magnesiumoxide, alloys of alumina, zirconia or magnesium oxide over the entiresurface of the object.
 10. The method set forth in claim 3 wherein step(b) comprises depositing, using an ion-beam deposition (IBD) technique,successive layers of alumina, zirconia, magnesium oxide, alloys ofalumina, zirconia or magnesium oxide over selected portions of theobject.
 11. A method of applying a ceramic insulation layer over adevice or component to be insulated comprising the steps of: (a)sputtering a layer of titanium of at least about 300 Å thick over thoseportions of the device or component which are to be insulated; (b)applying a layer of photosensitive polyamide onto a surface of thedevice or component which is to be insulated; (c) applying a mask thatexposes those areas on the surface of the device or component wherealumina is not to be applied; (d) shining ultra violet (UV) lightthrough the mask to polymerize the polyamide; (e) washing off anyunpolymerized polyamide with xylene or an equivalent substance; (f)depositing a layer of alumina having a thickness of between 4 and 10microns on the surface of the device or component which is to beinsulated; (g) breaking or scribing the alumina that was deposited overthe polyamide; (h) soaking the device or component in a pyrana solution(H₂SO₄×4+H₂O₂×2 heated to 60°) for 30 to 60 minutes; and (i) lifting offthe polyamide, thereby removing alumina from those areas on the surfaceof the device or component in those areas where alumina is not wanted.